1. Field of the Invention
The present invention relates to an analog-to-digital converter (hereinafter referred to as "AD converter") formed within a semiconductor integrated circuit, particularly, to an AD converter in which a current supply path is provided exclusively for each constituent block of the AD converter and an analog voltage comparator, which is one of said constituent blocks, is positioned closer to the current supply terminal than the other constituent blocks.
2. Description of the Related Art
AD converters of various types are known to the art including, for example, the integration type, flush type and sequential comparison type, which are described in, for example, ANDREW G. F. DINGWALL (RCA Laboratories), "Monolithic Expandable 6 Bit 20 MHz CMOS/SOS/ A/D Converter" IEEE J of S.S.C. Vol. SC-14, No. 6P926-932; August, 1979. These AD converters comprise a DA converter for producing a reference voltage, an analog voltage comparator for comparing an analog input voltage with the reference voltage or a group of analog voltage comparators and, as desired, a control circuit for controlling the DA converter and the analog voltage comparator.
In the conventional AD converter formed in a chip of a semiconductor integrated circuit, current is supplied from the current supply terminal of a power source to the control circuit for controlling the analog voltage comparator or comparators and the DA converter via a common current supply path. For example, the conventional AD converter is constructed as shown in FIG. 1. As seen from the drawing, an analog voltage 3, a DA converter 4, a control circuit 5 (in this case, a sequential comparative register), and common current supply paths 6, 7 are provided between current supply terminals 1 and 2.
In the AD converter, it is important for the analog voltage comparator to perform comparison accurately between the reference voltage and the analog input voltage. In the prior art shown in FIG. 1, however, a change in current within the DA converter 4 and a control circuit, which accompanies the switching operation of the DA converter 4 and the control circuit, gives an influence to the analog voltage comparator 3 via the current supply paths 6, 7. As a result, the comparing operation of the analog voltage comparator 3 becomes unstable, leading to a low accuracy in the AD conversion.
In forming an AD converter on a chip of a semiconductor integrated circuit, no special attention was paid in the past to the locational relationship between the circuit block of the analog voltage comparator 3 and the other circuit blocks such as the circuit block of the DA converter 4. It should be noted in this connection that the current supply paths 6, 7 leading from the current supply terminals 1, 2 to the analog voltage comparator 3 become the longer, as the distance of the comparator 3 from the terminals 1, 2 becomes the longer, resulting in an increased parasitic inductance of the current paths 6 and 7. It follows that a change in voltage is increased when current flows through these current supply paths 6, 7. It should be noted that the operation of the analog voltage comparator 3 is likely to be affected by, particularly, a change in the power source voltage. Naturally, a big change in voltage caused by the parasitic inductance noted above leads to an unstable operation of the analog voltage comparator 3.